Can you confirm this is infill and not designed in ribing.
If infill what settings are you using. Rectilinear would be a continuouse extrusion from one edge to the other.
if its designed ribing then the extusion would be between each rib so the issue may be at the start or end of a run. (Retraction, acceleration settings etc).

It is indeed Curas default infill, and the same g-code on my other machines does not produce these gaps.
The machine in question is a rather hacked repstrap build, so it can be almost anything there not very well designed/built. Which is why I ask for some suggestions on where to look. The finished prints look as good as one would expect from a machine like that, it is just the infill that is rather odd.

Did you change the steper on the extruder?
Its increadably consistant. Which direction is the head moving when it leaves gaps? (In picture, left to right or right to left, or both). Looks like some walls have the issue in the opposite direction but doesn't have a complete break. I assume the solid infill is OK.
Is there any retraction going on.
Have you reset the current seting on the drivers?
If its the same Gcode etc all i can think of is that the extruder steps are not regular. Sort of has a missing step but i can't seehow that could not happen on solid infill and be at the same linear distance on each run.

QuoteMCcarman
Its increadably consistant. Which direction is the head moving when it leaves gaps? (In picture, left to right or right to left, or both). Looks like some walls have the issue in the opposite direction but doesn't have a complete break.

I dont know, I tried to repeat it today, but I had to slice the file again, and of course Cura this time decided to plan the moves a little different. I still have the gaps nut not as nice as the one you have seen, and not in the same places. I have tried to stop the part cooling fan, it blows over the part in one direction only. Didn't make much difference.

QuoteMCcarman
I assume the solid infill is OK.

Haven't tried solids.

QuoteMCcarman
Is there any retraction going on.

Retraction is not needed here. The machine is a Bowden setup.

QuoteMCcarman
Have you reset the current seting on the drivers?

If its the same Gcode etc all i can think of is that the extruder steps are not regular. Sort of has a missing step but i can't see how that could not happen on solid infill and be at the same linear distance on each run.

Agree, if my extruder would be loosing steps they would not be that consistent. And I would hear it.

I think I will increase extruder flow a little to see if it makes any difference. Also I could try to go a lot slower.

Hi Mogul.
Just wanted to be clear if you changed the extruder steper. The OP said you changed the stepers and then the problem started. But we tend to think of the X, Y, z stepers and forget the extruder. So i wanted to be clear if it changed. If it didn't change then that should not be the problem.
I wanted to confirm if solid infill was an issue although I assume the solid (first layer) is OK as you haven't mentioned it.
If its bowden then i would expect you have some retraction but it shouldn't be retracting part way across the infill. But its so consistent i would expect to see this if it was doing a lift and retract over the infill patern.
Notice that the infill has a blob at the gap. So either the movement slows/stops or the extruder starts at the blob. So depends which direction the heads moving. If the extruder was just stopping early I would expect it to taper off with some oozing.
I assume that the perimeters are pure X and Y. I was thinking it only occurs on diagonal movement when both motors are operating but there is no issue on the 1st layer infill reported. Might be worth doing a print with the model rotated 45 degrees to confirm it.
Since the GCODE is the same did you change any settings on the controller ?

Pretty stumped here. You could try swaping the steppers around to see if that has any effect. Z should be OK so its just X, Y and extruder.

Just wanted to be clear if you changed the extruder steper. The OP said you changed the stepers and then the problem started. But we tend to think of the X, Y, z stepers and forget the extruder. So i wanted to be clear if it changed. If it didn't change then that should not be the problem.

And you are correct, it changed the X and Y steppers, Z and extruder is the same as always.

QuoteMCcarman
I wanted to confirm if solid infill was an issue although I assume the solid (first layer) is OK as you haven't mentioned it.

First and last layers are both fine, the part on the photo I stopped early to let you guys see.

QuoteMCcarman
If its bowden then i would expect you have some retraction but it shouldn't be retracting part way across the infill. But its so consistent i would expect to see this if it was doing a lift and retract over the infill patern.

And I think it should have no reason to do retract nor z-lift when infill crossings.

QuoteMCcarman
Notice that the infill has a blob at the gap. So either the movement slows/stops or the extruder starts at the blob. So depends which direction the heads moving. If the extruder was just stopping early I would expect it to taper off with some oozing.

I have noticed those blobs too, can't explain them.
I repeated the test yesterday, with an orange filament, I stopped the cooling fan half way through. I got the same gaps as always, but when the cooling stopped the blobs got pulled over, so the gaps turned into stringing.

QuoteMCcarman
I assume that the perimeters are pure X and Y. I was thinking it only occurs on diagonal movement when both motors are operating but there is no issue on the 1st layer infill reported. Might be worth doing a print with the model rotated 45 degrees to confirm it.

Again you guessed it right, the box was placed with the outer walls on pure X and Y. Good idea trying to rotate the part 45deg.

QuoteMCcarman
Since the GCODE is the same did you change any settings on the controller ?

Not really, both printers run marlin, various versions. I had to recompile the firmware to reconfig some endstops, same code, but probably a newer version of the arduino environment.

QuoteMCcarman
Pretty stumped here.

So am I...

I have a list of things to try now:

different filament

different temperature

rotate part

much lower speed

different infill setting

One thing more, the machine with the trouble is tethered to octoprint on a Pi3, whereas the reference machine prints from SD card, can it be I empty the buffer in marlin? I expect not, would think I would hear it making noises if the movement was stopping all the time on the diagonal infill lines.

I would double check the firmware. Probably do another compile and flash. Do an errase 1st.
I can't envisage anything mechanical causing this.
Since the GCODE is OK i would be looking for some thing in the firmware making it do this.
I don't think its the buffer as that just holds the GCODE instruction which should be "go from perimeter to perimeter at speed X + Extrude Y amount at speed Z". Nothing here to interupt the extrusion. Could be some other interuption though.

Worth trying the code from an SD card.

The outer part of the brim- does that have blobs that line up with the infill ends? Looks like it may in the picture. No idea if that is relevant.